The Sky Sports pundit was damning in his appraisal at half-time, analysing the moment De Gea cleared straight to Everton striker Dominic Calvert-Lewin, with the ball ricocheting into the net.
Asked how he would have reacted had he been De Gea's team-mate or manager, he said: "As a manager or player I'd kill him! You'd kill him!
"He's a goalkeeper being too clever. He just takes too long. What is he waiting for?!
"If you're going to make a decision at least do it quickly. These are huge moments for United in terms of trying to get into the top four, and you've got your experienced goalkeeper... what is he waiting for?!
I think there's almost a bit of arrogance to it, 'look at me...'
"I would have lynched him at half-time. I'd have no time for that carry on."
United were able to comeback just before half time through another goalkeeping error.
'De Gea is best keeper in world' - Solskjaer defends De Gea
New signing Bruno Fernandes advanced at goal before cutting across the ball with a long-range shot, which bent around the outstretched arms of England international Pickford.
Keane was similarly unimpressed, saying: "I don’t care if he’s the England number one. He’s not a good goalkeeper.”
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